Staged lean combustion for rapid start of a fuel processor

ABSTRACT

A fuel processor for rapidly achieving operating temperature during startup. The fuel processor includes a reformer, a shift reactor, and a preferential oxidation reactor is provided for deriving hydrogen for use in creating electricity in a plurality of fuel cells. A first combustion heater system is coupled to at least one of the reformer, the shift reactor, and the preferential oxidation reactor to preheat the component during a rapid startup sequence. That is, the first combustion heater system is operable to produce thermal energy as a product of the combustion of air and fuel in the form of a first heated exhaust stream. This first heated exhaust stream is then used to heat the component directly or by using a heat exchanger type system. The first heated exhaust stream is also used by a second combustion device as a source of oxygen or diluent.

FIELD OF THE INVENTION

The present invention generally relates to fuel processors and, moreparticularly, relates to a fuel processor having a two-stage leancombustion system for rapid start of the fuel processor.

BACKGROUND OF THE INVENTION

H₂—O₂ fuel cells use hydrogen (H₂) as a fuel and oxygen (as air) as anoxidant. The hydrogen used in the fuel cell can be derived from thereformation of a hydrocarbon fuel (e.g. methanol or gasoline). Forexample, in a steam reformation process, a hydrocarbon fuel (such asmethanol) and water (as steam) are ideally reacted in a catalyticreactor (a.k.a. “steam reformer”) to generate a reformate gas comprisingprimarily hydrogen and carbon monoxide.

An exemplary steam reformer is described in U.S. Pat. No. 4,650,727 toVanderborgh. For another example, in an autothermal reformation process,a hydrocarbon fuel (such as gasoline), air and steam are ideally reactedin a combined partial oxidation and steam reforming catalytic reactor(a.k.a. autothermal reformer) to generate a reformate gas containinghydrogen and carbon monoxide. An exemplary autothermal reformer isdescribed in U.S. application Ser. No. 09/626,553 filed Jul. 27, 2000.The reformate exiting the reformer contains undesirably highconcentrations of carbon monoxide most of which must be removed toprevent poisoning of the catalyst of the fuel cell's anode. In thisregard, carbon monoxide (i.e., about 3-10 mole %) contained in theH₂-rich reformate/effluent exiting the reformer must be reduced to verylow nontoxic concentrations (i.e., less than about 20 ppm) to avoidpoisoning of the anode.

It is known that the carbon monoxide, CO, level of thereformate/effluent exiting a reformer can be reduced by utilizing aso-called “shift” reaction wherein water (i.e. steam) is added to thereformate/effluent exiting the reformer, in the presence of a suitablecatalyst. This lowers the carbon monoxide content of the reformateaccording to the following ideal shift reaction:CO+H₂O→CO₂+H₂.

Some (i.e., about 0.5 mole % or more) CO still survives the shiftreaction. Hence, shift reactor effluent comprises hydrogen, carbondioxide, water carbon monoxide, and nitrogen.

The shift reaction is not enough to reduce the CO content of thereformate enough (i.e., to below about 20-200 ppm). Therefore, it isnecessary to further remove carbon monoxide from the hydrogen-richreformate stream exiting the shift reactor, and prior to supplying it tothe fuel cell. It is known to further reduce the CO content of H₂-richreformate exiting the shift reactor by a so-called “PrOx” (i.e.,preferential oxidation) reaction effected in a suitable PrOx reactoroperated at temperatures which promote the preferential oxidation of theCO with air in the presence of the H₂, but without consuming/oxidizingsubstantial quantities of the H₂ or triggering the so-called “reversewater gas shift” (RWGS) reaction. The PrOx and RWGS reactions are asfollows:CO+½O₂→CO₂(PrOx)CO₂+H₂→H₂O+CO (RWGS)The PrOx process is described in a paper entitled “Methanol FuelProcessing for Low Temperature Fuel Cells” published in the Program andAbstracts of the 1988 Fuel Cell Seminar, Oct. 23-26, 1988, Long Beach,Calif., and in Vanderborgh et al U.S. Pat. No. 5,271,916, inter alia.

Desirably, the O₂ required for the PrOx reaction will be about two timesthe stoichiometric amount required to react the CO in the reformate. Ifthe amount of O₂ exceeds about two times the stoichiometric amountneeded, excessive consumption of H₂ results. On the other hand, if theamount of O₂ is substantially less than about two times thestoichiometric amount needed, insufficient CO oxidation may occur andthere is greater potential for the RWGS reaction to occur. Accordinglyin practice, many practitioners use about 4 or more times thestoichiometric amount of O₂ than is theoretically required to react withthe CO.

PrOx reactors may be either (1) adiabatic (i.e. where the temperature ofthe reactor is allowed to rise during oxidation of the CO) or (2)isothermal (i.e. where the temperature of the reactor is maintainedsubstantially constant during oxidation of the CO). The adiabatic PrOxprocess is sometimes effected via a number of sequential stages, whichprogressively reduce the CO content in stages, and requires carefultemperature control, because if the temperature rises too much, the RWGSreaction can occur which counter productively produces more CO. Theisothermal process can effect the same CO reduction as the adiabaticprocess, but in fewer stages and without concern for the RWGS reactionif (1) the reactor temperature can be kept low enough, and (2) O₂depletion near the end of the reactor can be avoided.

One known isothermal reactor is essentially a catalyzed heat exchangerhaving a thermally conductive barrier or wall that separates the heatexchanger into (1) a first channel through which the H₂-rich gas to bedecontaminated (i.e. CO removed) passes, and (2) a second channelthrough which a coolant flows to maintain the temperature of the reactorsubstantially constant within a defined working range. The barrier wallhas a catalyzed first surface confronting the first channel forpromoting the CO+O₂ reaction and an uncatalyzed second surfaceconfronting the second channel for contacting the coolant therein toextract heat from the catalyzed first surface through the barrier. Thecatalyzed surfaces of adjacent barriers oppose each other, and areclosely spaced from each other, so as to define a narrow first channelthrough which the H₂-rich gas moves.

The reformation process of gasoline or other hydrocarbons operate athigh temperatures (i.e. about 600-800° C.). The water gas shift reactoris active at temperatures of 250-450° C., The PrOx reaction is active attemperatures of 100-200° C. Thus, it is necessary that the reformer, thewater gas shift (WGS) reactor, and the PrOx reactor are each heated totemperatures sufficient for the fuel processor to operate. Duringstart-up, however, a conventional fuel processor is such that theheating of various components is staged. This approach can lead toundesirable lag time for bringing the system on line. Alternately,external electrical heat sources (i.e. heaters) may be employed to bringthe components to proper operating temperatures. This approach requiresan external source of electricity such as a battery.

Accordingly, there exists a need in the relevant art to provide a fuelprocessor that is capable of heating the fuel processor componentsquickly to achieve these high operating temperatures for startup.Furthermore, there exists a need in the relevant art to provide a fuelprocessor that maximizes this heat input into the fuel processor whileminimizing the tendency to form carbon monoxide. Still further, thereexists a need in the relevant art to provide a fuel processor capable ofheating the fuel processor while minimizing the use of electrical energyduring startup and the reliance on catalytic reactions.

SUMMARY OF THE INVENTION

According to the principles of the present invention, a fuel processorfor rapidly achieving operating temperature during startup is providedhaving an advantageous construction. The fuel processor includes areformer, a shift reactor, and a preferential oxidation reactor forderiving hydrogen for use in creating electricity in a plurality ofH₂—O₂ fuel cells. A first combustion heater system is coupled to atleast one of the reformer, the shift reactor, and the preferentialoxidation reactor to preheat the component(s) during a rapid startupsequence. That is, the first combustion heater system is operable toproduce thermal energy as a product of the combustion of air and fuel inthe form of a first heated exhaust stream. This first heated exhauststream is then used to heat the component either directly or by using aheat exchanger type system.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a schematic view illustrating a fuel processor according to afirst embodiment of the present invention;

FIG. 2 is a schematic illustration of a second combustion burner system;

FIG. 3 is a schematic view illustrating a fuel processor according to asecond embodiment of the present invention;

FIG. 4 is a schematic view illustrating a fuel processor according to athird embodiment of the present invention; and

FIG. 5 is a schematic view illustrating a fuel processor according to afourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiments is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses. For example, the present invention ishereafter described in the context of a fuel cell fueled by reformedgasoline. However, it is to be understood that the principles embodiedherein are equally applicable to fuel cells fueled by other reformablefuels.

Referring to FIG. 1, a fuel processor, generally indicated as 10,according to a first embodiment of the present invention is illustrated.Fuel processor 10 generally includes a first burner system 12, an autothermal reformer (reformer) 14, a heat exchanger 16, a water gas shiftreactor/heat exchanger (WGS/HX) 18, a preferential oxidation/vaporizer(PrOx/vaporizer) reactor 20, a fuel cell stack 22, a second burnersystem 24, and a catalytic combustor (comb) reactor 26.

First burner system 12 and second burner system 24 are primarily used toheat the components of fuel processor 10 during a startup cycle toachieve rapidly and efficiently an optimal operational temperaturewithin fuel processor 10. First burner system 12 and second burnersystem 24 may burn various types of fuels, such as but not limited tohydrocarbons or hydrogen. Following such, fuel processor 10 is thencapable of efficiently producing electrical energy through thecombination of hydrogen and oxygen according to known fuel celltechnology. First burner system 12 and second burner system 24 are eachof either a premixed or diffusion-type burner that produces heat throughinternal combustion. The rate of heating is determined by the heatingvalue of the fuel that is burned. The amount of fuel burned is dependentupon the air stream rate at near overall stoichiometric conditions.First burner system 12 and second burner system 24 could be thermal orcatalytic in design.

Preferably, combustion heating occurs in two stages, namely within firstburner system 12 and second burner system 24, to minimize the initialtemperature of the gases that are necessary to efficiently and quicklyheat reformer 14, WGS/HX 18, PrOx/vaporizer reactor 20, and a catalyticcombustor 26 to operating temperature. That is, reformer 14, WGS/HX 18,PrOx/vaporizer reactor 20, and catalytic combustor 26 are eachsusceptible to damage if exposed to excessive temperature. However, inorder to heat these components to a predetermined operating temperaturewith a single burner, it is necessary that the output gases of thesingle burner be sufficiently heated initially to carry enough heatdownstream to heat the remaining components. Therefore, the output gasesof the single burner may pose a risk to upstream components since thetemperature may be above that which the upstream component is capable oftolerating. Accordingly, it is preferable to employ a two stage heatingsystem to effectively heat all components during start up withoutexposing such components to excessive temperature. Combustion in twostages also serves to reduce the maximum flame temperature, whichreduces the production of undesirable NO_(x) formation. Alternatively,combustion could occur in more than two stages for improved localcontrol of the resultant heating. For example, an additional burnercould be used to heat WGS/HX 18 and PrOx/vaporizer reactor 20 directly.

As best seen in FIG. 1, fuel processor 10 is arranged such that firstburner system 12 includes a first air inlet stream 28 and a first fuelinlet stream 30. First air inlet stream 28 may be obtained as a directfeed from a system air compressor (not shown) or from the air feed 34 tofuel cell stack 22. The use of air from the air feed 34 to fuel cellstack 22 may provide additional flow rates to achieve higher heatingcapacity, if required.

The heated exhaust stream of first burner system 12, generally indicatedas 32, exits first burner system 12 as a fuel lean combustion exhaust toheat the downstream components of fuel processor 10. The particulartemperature of first burner exhaust stream 32 is preferably sufficientto heat the catalyst within reformer 14 to its optimized operatingtemperature, typically in the range of about 600-800° C. forhydrocarbons, such as gasoline. To this end, first burner system 12 ispreferably a premixed or diffusion-type and includes a high temperaturezone for flame stability. It is contemplated that first burner exhauststream 32 of first burner system 12 may be diluted with downstream air(not shown) to control the first burner exhaust stream to a temperaturesuitable to heat the catalyst within reformer 14. This downstream airmay be obtained by diverting a portion of first air inlet stream 28within first burner system 12 or may be obtained by utilizing anotherair source (i.e. reformer air 36). However, first air inlet stream 28 ispreferably obtained as a direct feed from a compressor or fromtemporarily bypassing fuel cell stack 22 and using air supply from astack air inlet stream 34 to achieve a high flow rate of air. Thisarrangement, as illustrated in FIG. 1, minimizes the pressure dropthrough fuel processor 10 by preventing air from first air inlet stream28 from passing through heat exchanger 16 during startup and, further,by preventing a reformer air inlet stream 36 or a reformer steam 38 frompassing through first burner system 12 during normal operation.

Accordingly, first burner exhaust stream 32 from first burner system 12sequentially heats a reformer inlet zone 40, reformer 14, heat exchanger16, and a sulfur trap 42. A bypass valve 44 is opened and a WGS valve 46is closed such that first burner exhaust stream 32 bypasses WGS/HX 18and flows to catalytic combustor 26 and second burner system 24.However, it should be understood that bypass valve 44 and WGS valve 46might be replaced with a single three-way valve (see FIG. 3). However,this two-valve arrangement enables bypass valve 44 to be located awayfrom the high temperature of reformate gas stream 54. Therefore, bypassvalve 44 may be made of lower temperature, better sealing materials toeliminate any leaks of reformate to catalytic combustor 26, which maylead to a loss to system efficiency.

It is believed that a brief description of the remaining components andconnections of fuel processor 10 is beneficial to adequately describethe startup procedures and components. Hence, with reference to a“normal” operation (e.g. after the system has started up and isrunning), reformer inlet zone 40 includes a reformer fuel inlet stream48, such as gasoline, and reformer air and inlet flow 50 from heatexchanger 16 to produce an inlet stream 52. Inlet stream 52 entersreformer 14 and catalytically reacts the fuel from reformer fuel inletstream 48 and air and water from reformer inlet flow 50 to form aH₂-rich reformate gas stream 54. Reformate gas stream 54 passes throughheat exchanger 16, which removes excess heat generated during thereformation cycle from reformate gas stream 54. This heat is then usedby heat exchanger 16 to heat a mixture of reformer air inlet stream 36and reformer steam 38 to produce reformer inlet flow 50. Reformate gasstream 54 then passes through sulfur trap 42 to remove sulfur and otherhydrocarbons and upon exit mixes with a water flow 56 to control thetemperature into WGS/HX 18 and further to humidify the effluent.

During normal operation, WGS valve 46 is open such that the humidifiedreformate gas stream 54 passes therethrough to WGS/HX 18 andPrOx/vaporizer reactor 20. As mentioned above, WGS/HX 18 is a water gasshift reactor and heat exchanger combination system. The heat exchangerportion of WGS/HX 18 is fluidly separate from the water gas shiftreactor portion to enable efficient heating of the shift reactorcatalyst during the startup procedure.

PrOx/vaporizer reactor 20 is a preferential oxidation reactor and avaporizer system. The vaporizer portion of PrOx/vaporizer reactor 20 isused as a heat exchanger to remove excess heat from the preferentialoxidation reaction and to produce reformer steam 86 and separate thereaction catalysts from the steam flow. WGS/HX 18 and PrOx/vaporizerreactor 20 are used to reduce CO-level therein to acceptable levels. TheCO-depleted, H₂-rich reformate stream 58 is then fed into the anode sideof fuel cell stack 22. Simultaneously, oxygen from stack air inletstream 34 is fed into the cathode side of fuel cell stack 22. Thehydrogen from reformate stream 58 reacts with the oxygen from stack airinlet stream 34 across a membrane electrode assembly to produceelectricity. Anode exhaust or stack effluent 60 from the anode side offuel cell stack 22 includes a portion of hydrogen that is directed backto catalytic combustor 26 to provide heat. Cathode exhaust 62 from thecathode side of fuel cell stack 22 includes oxygen also for use incatalytic combustor 26. The flow of cathode exhaust 62 to catalyticcombustor 26 is controlled via a pair of control valves, namely acombustor air control valve 64 and a cathode exhaust back pressure valve66. The closing of cathode exhaust back pressure valve 66 produces aback pressure that forces air through combustor air control valve 64 forcombustion in catalytic combustor 26. The opening of cathode exhaustback pressure valve 66 permits flow to an exhaust 67.

During a startup cycle, bypass valve 44 is opened and WGS valve 46 isclosed, thereby sending the lean gases of first burner system 12indirectly to second burner system 24. It should be understood that itis necessary to bypass WGS/HX 18 when lean combustion gases are flowingwithin fuel processor 10, since the oxygen within the combustion gasesmay react with the CuZn catalyst that is typically used in water gasshift (WGS) reactors. However, if WGS/HX 18 includes a nonpyrophoriccatalyst, bypass valve 44 and WGS valve 46 are not necessary and leancombustion gases may be permitted to flow along the normal operationpath through fuel cell stack 22 to second burner system 24 to simplifyfuel processor 10. Generally, it is undesirable to allow dry air to flowthrough fuel cell stack 22 for extended periods of time due to thedrying of the membranes typically used in PEM stacks. However, accordingto the principles of the present invention, if valves 44, 46 were notused, the resultant gas flow through fuel cell stack 22 is acceptablesince it contains moisture, which is a product of the lean combustionwithin first burner system 12 and very low carbon monoxide levels toprevent “poisoning” of the catalysts. If desired, a bypass valve may beused to bypass fuel cell stack 22.

Second burner system 24 is used to indirectly heat catalytic combustor26, WGS/HX 18, and PrOx/vaporizer reactor 20. Second fuel inlet stream68 is introduced downstream of catalytic combustor 26 and into secondburner system 24 such that during the combustion process, most of theremaining oxygen is consumed. However, it should be noted that it ispreferable to remain slightly fuel lean within second burner system 24to insure that unburned hydrocarbons are not present in the heatedexhaust stream 70. Second burner system 24 is preferably a premixed ordiffusion-type. More preferably, second burner system 24 is a premixedtype when used with liquid fuel to reduce the amount of emissionsproduced by the flame.

As best seen in FIG. 2, catalytic combustor 26 is indirectly heated.That is, under start conditions, combustor gas flow 88 to second burnersystem 24 is the product of lean combustion in first burner system 12flowing through bypass valve 44. Combustor gas flow 88 is indirectlyheated across a liner 202, which separates a flame 204 of second burnersystem 24 from combustor gas flow 88. Second fuel inlet stream 68 isadded to and mixes with combustor gas flow 88 after catalytic combustor26. For premix operation, second fuel inlet stream 68 is injected andmixes with the gas exiting the catalytic combustor 26 in a mixingchamber 206 before introduction into flame chamber 208. For diffusionoperation, there is no mixing in chamber 206 and second fuel inletstream 68 is injected downstream of a flame holder 210 and directly intoflame 204. For liquid fuel operation, the premixed approach is preferredso as to reduce the amount of emissions from flame 204. Flame holder 210may be of any conventional type, such as but not limited to a swirler,perforated plate (as shown in FIG. 2), backward facing step, bluff body,or transverse jets. Flame 204 can be initiated by spark plug 212.

As best seen in FIGS. 1 and 2, a spray vaporization zone 72 isdownstream from second burner system 24, which employs a spray waterstream 74 to reduce the gas temperature of exhaust stream 70 exitingsecond burner system 24 in the event the exit temperature of exhauststream 70 is too high for a start vaporizer 80 or WGS/HX 18. Thetemperature of exhaust stream 70 exiting second burner system 24 isreduced as it passes through spray vaporization zone 72 and startvaporizer 80 to produce exhaust stream 82. Exhaust stream 82 then flowsthrough the heat exchanger of WGS/HX 18 and a run vaporizer 96 toexhaust 67.

Thermal energy from second burner system 24 is also utilized byinitiating a start vaporizer water stream 78 though start vaporizer 80to produce a start vaporizer steam flow 76. Start vaporizer steam flow76 flows across the backside of PrOx/vaporizer reactor 20 to heatPrOx/vaporizer reactor 20, since the saturation temperature of startvaporizer steam flow 76 (134° C. at 3 atm.) complements the operatingtemperature of the catalyst within PrOx/vaporizer reactor 20. It shouldbe appreciated that utilizing the heat of vaporization can transfersignificant thermal energy. Drains for eliminating condensed water maybe incorporated to avoid the use of thermal energy to revaporize thecondensed water. PrOx/vaporizer reactor 20 is of a heat exchanger typeconstruction to separate the reaction catalyst from start vaporizersteam flow 76, a PrOx inlet water flow 84, and the resulting PrOx steamflow 86 that is generated.

PrOx steam flow 86 provides additional heating of heat exchanger 16, inaddition to the direct heat provided to heat exchanger 16 by firstburner system 12. If during the startup cycle the temperature of aWGS/HX exhaust exit flow 98 exceeds the vaporization temperature(typically about 150° C.), run vaporizer 96 can generate additionalsteam 102. That is, a run water 100 entering run vaporizer 96 isadjusted so that steam 102 is slightly superheated. Steam 102 joins withPrOx steam flow 86 to form reformer steam 38, which flows to heatexchanger 16. This process may be used to provide additional heating ofheat exchanger 16.

During a rapid start up cycle of fuel processor 10, full air flow isintroduced in first air inlet stream 28 of first burner system 12.Bypass valve 44 is opened and WGS valve 46 is closed so as to route aflow 90 to second burner system 24. Ignition members 212, such as sparkplugs, within first burner system 12 and second burner system 24 areenergized. Simultaneously, first fuel inlet stream 30 and second fuelinlet stream 68 are introduced to first burner system 12 and secondburner system 24, respectively, to start combustion. Alternate sequencymay be appropriate depending on the mechanization hardware. Alternatescenarios could light off at reduced flow or lead with first burnersystem 12 or second burner system 24. Confirmation of combustion withinfirst burner system 12 and second burner system 24 is obtained bysensors such as flame ionization or temperature measurement of firstburner exhaust stream 32 and at the exit of spray vaporization zone 72,respectively. First fuel inlet stream 30 is controlled to produce thedesired temperature for the catalyst within reformer 14 (typically about600-800° C.) for gasoline type hydrocarbons.

Second fuel inlet stream 68 is controlled to maintain near overallstoichiometric conditions to maximize the heat input to fuel processor10 for rapid startup. That is, the total fuel flow, which equals the sumof first fuel inlet stream 30 and second fuel inlet stream 68, reactsand consumes nearly all the oxygen provided by first air inlet stream 28to maximize the combustion heat produced without resulting in unburnedhydrocarbons.

Spray water stream 74 is introduced within spray vaporization zone 72 tomaintain the proper temperature of exhaust stream 82 through thevaporization of water so as not to exceed the temperature limits of thedownstream components. That is, spray water stream 74 ensures that startvaporizer 80, downstream from spray vaporization zone 72, is not exposedto excessively high temperatures (i.e. greater than about 600° C.).Moreover, spray water stream 74 ensures that exhaust stream 82 are notexcessively heated (typically less than about 300° C.) so as not todamage the CuZn type WGS catalyst. The control temperature may bealtered with the usage of precious metal based catalysts. A reduction ingas temperature further occurs across start vaporizer 80 due tovaporization of start vaporizer water stream 78. The quantity of startvaporizer water stream 78 is limited such that start vaporizer steamflow 76 is slightly superheated (typically about 150° C.). Furtherreduction in the quantity of start vaporizer water stream 78 is utilizedto favor heating WGS/HX 18 rather than PrOx/vaporizer reactor 20. Thestart operation is controlled as described above until fuel processor 10is heated to a predetermined temperature for normal operation.

Once the catalyst of PrOx/vaporizer reactor 20 and the catalyst ofWGS/HX 18 are above their minimum operation temperatures (typicallyabout 100° C. and about 220° C., respectively) and reformer steam 38 isflowing through heat exchanger 16, fuel processor 10 is ready to beginnormal operation. To determine whether such operating temperatures havebeen achieved, it is preferable to monitor and compare the temperatureof PrOx steam flow 86 to the operating temperature of PrOx/vaporizerreactor 20 and the temperature of WGS/HX exhaust exit flow 98 to theoperating temperature of WGS/HX 18. The availability of steam isdetermined by monitoring the temperature of reformer air and steam flow50. For high sulfur fuels, it is preferably that sulfur be removed fromthe liquid fuel or sulfur trap 42 be at its operating temperature(typically about 300-500° C.) to ensure full capacity such that sulfurdoes not pass to the catalyst of WGS/HX 18 or other downstreamcatalysts, as such catalyst may be damaged by the presence of sulfur.

Normal, fuel rich operation may be accomplished via several methods. Forinstance, fuel rich reformer flow for normal operation can beestablished by starting reformer fuel inlet stream 48 and reformer airinlet stream 36, and closing first air inlet stream 28 and first fuelinlet stream 30. Preferably, this transition occurs rapidly so as to notlinger at near stoichiometric conditions due to the excessively highassociated reaction temperatures. Moreover, this transition shouldpreferably occur quickly such that anatomic-oxygen-in-air-flow-to-carbon-in-fuel-flow ratio (oxygen-to-carbonratio) of less than one is not encountered, which may produceundesirable carbon.

Alternatively, normal, fuel rich operation may be established byinitially fully closing first fuel inlet stream 30 and first air inletstream 28 to first burner system 12. Reformer air inlet stream 36 andreformer fuel inlet stream 48 is then initiated in a way to preferablyavoid operation near stoichiometric conditions or oxygen-to-carbon ratioof less than one. However, operation in conditions where theoxygen-to-carbon ratio is less than one is permitted when steam flow isavailable. It is important to note that at least some steam flow isavailable after the start-up cycle from start vaporizer 80, runvaporizer 96, and PrOx/vaporizer reactor 20, which have all beenpreheated during the start-up cycle.

The change to fuel rich reformer operation is accompanied by the closingof second fuel inlet stream 68 and the addition of cathode exhaust 62 tocatalytic combustor 26 to complete combustion before exhaust. To thisend, catalytic combustor 26 must be kept sufficiently lean to maintainthe catalyst temperature below its operating limit (typically about 750°C.). To this point, reformer fuel inlet stream 48 is below its fullpower setting to insure that the operation of catalytic combustor 26 iswithin acceptable temperature limits (less than about 750° C.).

Once a stable flow through reformer 14 is established, WGS valve 46 isopened and bypass valve 44 is closed to direct reformate gas stream 54through the WGS portion of WGS/HX 18, the PrOx portion of PrOx/vaporizerreactor 20, and fuel cell stack 22. In conjunction with the changing ofvalve positions of bypass valve 44 and WGS valve 46, the flow of a PrOxair inlet stream 92 and PrOx inlet water flow 84 is initiated intoPrOx/vaporizer reactor 20. As fuel cell stack 22 draws current, thehydrogen content of anode exhaust stream 60 is greatly reduced, whichreduced the reaction temperature within catalytic combustor 26. Hence,spray water stream 74, if used, is controlled to produce the desiredtemperature until it can eventually be shut off. Start vaporizer waterstream 78 to start vaporizer 80 is also shut off and the control of theexit temperature of WGS/HX 18 is regulated using combustor air controlvalve 64, which controls the cathode exhaust stream 62 to catalyticcombustor 26.

If not initiated previously, run water 100 is initiated to run vaporizer96 once the temperature of WGS/HX exhaust exit flow 98 exceeds the watervaporization temperature (typically about 150° C.). The quantity of runwater 100 is determined within the energy availability of WGS/HX exhaustexit flow 98 to fully vaporize and the quantity of water available ordesired for operation. Run vaporizer steam flow 102 is available forreformer steam flow 38. Fuel processor 10 is now in a normal operatingmode for producing electricity.

For normal operation, reformer air inlet stream 36 to reformer fuelinlet stream 48 ratio is set to provide the desired reformer reactionexit temperature (typically about 750° C.). The temperature of reformategas stream 54 into the front end of WGS/HX 18 (typically about 250° C.)is controlled by the quantity of water flow 56 atomized into andvaporized by reformate gas stream 54.

PrOx air inlet stream 92 is set to provide the required carbon monoxideclean-up for fuel cell stack 22. Similarly, PrOx inlet water flow 84 isset to remove the associated heat release from PrOx/vaporizer reactor20. PrOx inlet water flow 84 is also set to provide single phase PrOxsteam flow 86 as indicated by temperature measurements of this stream.Alternate embodiments may adjust PrOx air and water flows to obtainoptimum performance. For example excess PrOx air may be used to handleCO spikes, or two phase PrOx steam flow may be used to provide thermalbalance. Run vaporizer steam flow 102 is used as a surplus to increasethe fuel processor efficiency or to meet transient steam flowrequirements of the system, but is not utilized for thermal balance. Theincreased steam 102 provided by run vaporizer 96 also reduces the carbonmonoxide level from reformer 14, which helps to minimize the exothermwithin WGS/HX 18. Steam 102 from run vaporizer 96 can also be used tomoderate variations in reformer steam 38 or the molar steam flow toatomic carbon in fuel flow ratio (steam-to-carbon ratio). Furthercontrol of overall steam flow can be achieved through the use ofPrOx/vaporizer reactor 20. That is, if additional steam flow is requiredduring transient operation, PrOx air inlet stream 92 can be increased toprovide additional thermal energy by exothermic reaction with reformategas stream 104 within PrOx/vaporizer reactor 20 to vaporize additionalwater delivered to PrOx/vaporizer reactor 20 via PrOx inlet water flow84.

To increase or decrease the output from fuel processor 10, reformer fuelinlet stream 48 is increased or decreased, respectively, and thecommensurate changes in air and water flows throughout the system aremade to maintain the system thermal balance and stoichiometry asdescribed above. The commensurate changes in air and water flows maylead or lag behind the changes in fuel flow to achieve the optimalresponse time and control within the desired reactor operating regimes.

It should be recognized that the present invention enables the uniquecapability to control the temperature of WGS/HX 18 and the ability tohandle unloads of fuel cell stack 22. The temperature of reformate 54before entering WGS/HX 18 is controlled using water flow 56. Primarycontrol of the temperature of WGS/HX 18 is provided by exhaust stream82. The temperature of exhaust stream 82 can be adjusted by the amountof cathode exhaust 62 directed to catalytic combustor 26 by combustorair control valve 64. According to the present embodiment, it isdesirable that WGS/HX 18 operate at nearly constant temperature andwithin the operating limits thereof. Preferably, as described above,CuZn catalyst is used within WGS/HX 18 wherein temperatures above about300° C. may damage the catalyst and temperatures below about 200° C.will greatly reduce the activity of the shift reaction. A challengeassociated with this narrow temperature window involves the removal ofheat generated by the exothermic reaction within WGS/HX 18, specificallyCO+H₂O→CO₂+H₂+thermal energy. By utilizing exhaust stream 82, which hasa relatively high mass flow rate and thus a high heat capacity ascompared to all of the available flow streams, the temperature rise(i.e. thermal energy) within WGS/HX 18 can be most effectivelyminimized. The ability to control the temperature of exhaust stream 82is also important to prevent quenching of the water gas shift reactionwith WGS/HX 18 (typically below about 220° C.), which may occur if thetemperature of exhaust stream 82 is too low. Indirect exothermicreduction is achieved by maximizing the quantity of reformer steam 38 toreformer 14, which lowers the carbon monoxide level entering WGS/HX 18.Maximizing the quantity of run water 100 vaporized in run vaporizer 96can also increase the flow of reformer steam 38. Still further,increasing the flow of PrOx air inlet stream 92 can increase the steamgenerating capacity of PrOx/vaporizer reactor 20. However, increasedflow of PrOx air inlet stream 92 may decrease the efficiency of fuelprocessor 10, which is not desirable for steady operation, but iseffective to increase steam generation during transient operations toavoid reducing the steam-to-carbon ratio or to limit the exotherm withinWGS/HX 18 to maintain the temperature of the WGS catalyst below damaginglevels.

At least three mechanisms are available for accommodating fuel cellstack 22 unloads. As used herein, the term unload refers to when theelectric current draw from fuel cell stack 22 is reduced. Specifically,these mechanisms include 1) increasing cathode exhaust 62, 2) initiatingor increasing flow of start vaporizer water stream 78 to start vaporizer80, and 3) injecting spray water stream 74 into spray vaporization zone72. When fuel cell stack 22 unloads, the hydrogen content of anodeexhaust stream 60 increases. This increase in hydrogen content willcause the temperature from catalytic combustor 26 to increase. Since itis necessary to control the temperature of WGS/HX 18, it is necessary tolimit or control the temperature of exhaust stream 70 from catalyticcombustor 26. Similarly, catalytic combustor 26 has an operatingtemperature limit of typically about 750° C. Exceeding this operatingtemperature limit may damage the catalyst material within combustor 26.

As a first measure, cathode exhaust 62 is controlled to cool thecatalyst of catalytic combustor 26 below its temperature limit. Thisenables exhaust stream 82 to be used to cool WGS/HX 18. If thetemperature of catalytic combustor 26 exceed the desired temperature ofWGS/HX 18 and all of the available cooling capacity of cathode exhaust62 is used, the temperature of exhaust stream 82 is lowered byinitiating or increasing flow of start vaporizer water stream 78 tostart vaporizer 80. This increases the steam-to-carbon ratio of fuelprocessor 10. Transition of water from run vaporizer 96 to startvaporizer 80 may be appropriate to dissipate excess heat in exhauststream 82, if excess water is not available. However, direct spray waterstream 74 may be used, particularly if the temperature into startvaporizer 80 exceeds its temperature limit (typically about 600° C.) orif an increased steam-to-carbon ratio in fuel processor 10 is notdesired. It is important to note that spray water stream 74 into exhauststream 70 is typically not recovered. Of course, as the load on fuelcell stack 22 decreases, the fuel flow and the overall fuel processoroutput is decreased to respond to the reduced power demand.

During a fuel processor shutdown, the electrical demand is reduced to aminimum predetermined level and fuel cell stack 22 is unloaded. Whenfuel cell stack 22 unloads, the H₂ content of anode exhaust stream 60increases and, thus, increased flow of cathode exhaust 62 throughcombustor air control valve 64 is required to limit the catalysttemperature of catalytic combustor 26. Bypass valve 44 is then openedand WGS valve 46 is closed to direct the reformate gas stream throughbypass valve 44 to catalytic combustor 26. PrOx air inlet stream 92 andPrOx inlet water flow 84 are also shut off. Still further, run water100, reformer air inlet stream 36, and reformer fuel inlet stream 48 areshut off. If desired, continuing the flow of cathode exhaust 62 over thebackside (or HX) of WGS/HX 18 can cool WGS/HX 18. This continued flowmay be desirable to reduce the temperature of the catalyst within WGS/HX18 to lower its catalytic activity in the event it becomes exposed toair leaks after shutdown. However, WGS valve 46 and check valves (suchas in anode exhaust 60) provide protection against air leaking intoWGS/HX 18.

As best seen in FIGS. 3-5, alternative embodiments of the presentinvention are illustrated, generally indicated as 10′, 10″ and 10′″;respectively. It should be noted that like reference numerals designatelike or corresponding parts throughout the several views.

Referring to FIGS. 3-5, a direct air inlet 302 is provided to secondburner system 24 to provide fresh combustion air (i.e. not exhaust gasfrom first burner system 12) to second burner system 24. By providingdirect combustion air to second burner system 24, the operationalstability of second burner system 24 may be improved. That is, thecombustion air is accurately controllable and, thus, the stability ofthe flame within second burner system 24 is improved. By improving thestability and thus the temperature control of second burner system 24,it may be possible to eliminate spray vaporization zone 72.

More particularly, as seen in FIGS. 3-5, a bypass flow 90′, 90″, 90′″from first burner system 12 is directed downstream of second burnersystem 24 and direct air inlet 302 is supplied to second burner system24 for combustion. To start the fuel processor, direct air inlet 302would be started and otherwise the startup procedure would be aspreviously described.

With particular reference to FIG. 3, since catalytic combustor 26 isupstream of bypass flow 90′, the transition to normal operation ispreferably modified to avoid sending reformate out exhaust 67.Specifically, the transition to normal operation would include closingfirst fuel inlet stream 30 and first air inlet stream 28. Reformer steam38, which is generated in part by start vaporizer 80, purges reformerinlet zone 40, reformer 14, heat exchanger 16, and sulfur trap 42 toremove air before opening the flow to WGSIHX 18. WGS valve 46 is thenopened and by-pass valve 44 is closed. Reformer air inlet stream 36 andreformer fuel inlet stream 48 are then started to begin fuel richoperation. Simultaneously, PrOx air inlet stream 92 and PrOx inlet waterflow 84 would be started. Second fuel inlet stream 68 and direct airinlet 302 would then be stopped. Anode exhaust stream 60 and cathodeexhaust 62 would then be reacted in combustor 26. As fuel cell stack 22draws current, the hydrogen content of anode exhaust stream 60 isgreatly reduced and the reaction temperature in combustor 26 drops.Accordingly, spray water stream 74 and start vaporizer water stream 78may be shut off. Thus, fuel processor 10′ is now in the normal operatingmode.

With particular reference to FIG. 4, it should briefly be noted thatcombustor 26 may be positioned downstream of second burner system 24such that bypass flow 90″ may enter combustor 26 without flowing throughsecond burner system 24. Furthermore, with particular reference to FIG.5, combustor 26 may be positioned along side second burner system 24,thereby modifying bypass line 90′″. This arrangement has the advantagethat combustor 26 is not exposed to exhaust stream 70 from second burnersystem 24 or spray water stream 74 from spray vaporization zone 72.

According to the principles of the present invention, a fuel processoris provided that is capable of heating the fuel processor componentsquickly to achieve proper operating temperatures for startup.Furthermore, the fuel processor of the present invention maximizes thisheat input into the fuel processor while minimizing the tendency to formcarbon. Still further, the fuel processor of the present inventionprovides a fuel processor capable of heating the fuel processorcomponent while minimizing the use of electrical energy during startupand the reliance on catalytic reactions. It should be readilyappreciated by those skilled in the art that the present inventionenables the potential usage of inexpensive CuZn catalyst only withoutthe need for an additional coolant loop. Moreover, the tight control oftemperatures within the fuel processor, which is afforded by the presentinvention, enables the optimization of reactor size and catalyst usageresulting in reduced active metal costs. Still further, the presentinvention provides improved transient carbon monoxide concentrationperformance.

The description of the invention is merely exemplary in nature and,thus, variations are not to be regarded as a departure from the spiritand scope of the invention.

1. A fuel processor for rapidly achieving operating temperature, saidfuel processor comprising: a reformer converting a hydrogen-containingfuel to H₂-containing reformate; a shift reactor in fluid communicationwith said reformer, said shift reactor being operable to reduce carbonmonoxide levels of said reformate; a preferential oxidation reactor influid communication with said shift reactor, said preferential oxidationreactor being operable to further reduce carbon monoxide levels of saidreformate exiting said shift reactor; and a first combustion heatersystem coupled to at least one of said reformers said shift reactor, andsaid preferential oxidation reactor, said first combustion heater systemoperated in a lean state to produce thermal energy as a product ofinternal combustion of air and fuel in the form of a first heatedexhaust stream that is passed through said at least one of saidreformer, said shift reactor, said preferential oxidation reactor; asecond combustion heater system coupled to at least another of saidreformer, said shift reactor, and said preferential oxidation reactor,said second combustion heater system operated to produce thermal energyas a product of the combustion of air and fuel in the form of a secondheated exhaust stream that is passed through said at least another ofsaid reformer, said shift reactor, and said preferential oxidationreactor.
 2. The fuel processor according to claim 1, wherein said secondcombustion heater further comprises an air inlet providing fresh airthereto.
 3. The fuel processor according to claim 1, wherein said secondcombustion heater system is positioned in series with said firstcombustion heater system.
 4. The fuel processor according to claim 3,wherein said second combustion heater further comprises a first inletproviding fresh air thereto and a second inlet in fluid communicationwith said first heated exhaust stream.
 5. The fuel processor accordingto claim 4, wherein said first heated exhaust stream is used to dilutesaid fresh air to control the temperature of said second heated exhauststream.
 6. The fuel processor according to claim 1, further comprising:a control valve system selectively diverting said first heated exhauststream from said first combustion heater system from passing throughsaid shift reactor.
 7. The fuel processor according to claim 1, furthercomprising: a heat exchanger operatively associated with at least one ofsaid reformer, said shift reactor, and said preferential oxidationreactor, said heat exchanger being exposed to at least one of said firstheated exhaust stream and said second heated exhaust stream for heatingsaid at least one of said reformer, said shift reactor, and saidpreferential oxidation reactor.
 8. The fuel processor according to claim1, further comprising: a spray vaporization zone coupled downstream fromsaid second combustion heater system, said spray vaporization zone beingoperable to maintain a predetermined temperatures of said second heatedexhaust stream.
 9. The fuel processor according to claim 1, furthercomprising: a control valve system selectively routing an O₂-containingcathode effluent from a fuel cell stack to a catalyst combustor and saidsecond combustion heater system.
 10. The fuel processor according toclaim 9, wherein said control valve system comprises: a combustor aircontrol valve selectively routing one of a group consisting of air andsaid O₂-containing cathode effluent to said catalyst combustor; and acathode back pressure valve selectively applying a fluid back pressureto facilitate routing of said O₂-containing cathode effluent to saidcatalyst combustor.
 11. The fuel processor according to claim 1, whereinsaid combustion of said air and said fuel in said first combustionheater system is lean of stoichiometric condition and said combustion ofsaid air and said fuel in said second combustion heater system isgenerally near ideal stoichiometric condition.
 12. The fuel processoraccording to claim 1, further comprising: a catalyst combustorpositioned in series upstream from said second combustion heater system.13. The fuel processor according to claim 1, further comprising: acatalyst combustor positioned in series downstream from said secondcombustion heater system.
 14. The fuel processor according to claim 1,further comprising: a catalyst combustor positioned such that an outputof said catalyst combustor is input downstream of said second combustionheater system.
 15. A fuel processor comprising: a reformer converting ahydrogen-containing fuel selected from the group consisting of alcoholand hydrocarbons to H₂-containing reformate; a shift reactor in fluidcommunication with said reformer, said shift reactor being operable toreduce carbon monoxide levels of said reformate; a preferentialoxidation reactor in fluid communication with said shift reactor, saidpreferential oxidation reactor being operable to further reduce carbonmonoxide levels of said reformate exiting said shift reactor; a firstcombustion heater system coupled to at least one of said reformer, saidshift reactor, and said preferential oxidation reactor, said firstcombustion heater system operated in a lean state to produce thermalenergy as a product of internal combustion in the form of a first heatedexhaust stream that is passed through said at least one of saidreformer, said shift reactor, and said preferential oxidation reactor;and a second combustion heater system coupled to at least another ofsaid reformer, said shift reactor, and said preferential oxidationreactor, said second combustion heater system operated in a slightlylean state to produce thermal energy as a product of combustion in theform of a second heated exhaust stream that is passed through said atleast another of said reformer, said shift reactor, and saidpreferential oxidation reactor.
 16. The fuel processor according toclaim 15, wherein said second combustion heater further comprises an airinlet providing fresh air thereto.
 17. The fuel processor according toclaim 15, wherein said second combustion heater system is positioned inseries with said first combustion heater system.
 18. The fuel processoraccording to claim 17, wherein said second combustion heater furthercomprises a first inlet providing fresh air thereto and a second inletin fluid communication with said first heated exhaust stream.
 19. Thefuel processor according to claim 18, wherein said first heated exhauststream Is used to dilute said fresh air to control the temperature ofsaid second heated exhaust stream.
 20. The fuel processor according toclaim 17, further comprising: a first control valve system selectivelyrouting said first heated exhaust stream to said second combustionheater system during a startup cycle.
 21. The fuel processor accordingto claim 15, further comprising: a spray vaporization zone coupleddownstream from said second combustion heater system, said sprayvaporization zone being operable to maintain a predeterminedtemperatures of said second heated exhaust stream.
 22. The fuelprocessor according to claim 15, further comprising: a second controlvalve system selectively routing an O₂-containing cathode effluent froma fuel cell stack to a catalyst combustor and said second combustionheater system.
 23. The fuel processor according to claim 22, whereinsaid second control valve system comprises: a combustor air controlvalve selectively routing said O₂-containing cathode effluent to saidcatalyst combustor; and a cathode back pressure valve selectivelyapplying a fluid back pressure to facilitate routing of saidO₂-containing cathode effluent to said catalyst combustor.
 24. The fuelprocessor according to claim 15, further comprising: a catalystcombustor positioned in series upstream from said second combustionheater system.
 25. The fuel processor according to claim 15, furthercomprising: a catalyst combustor positioned in series downstream fromsaid second combustion heater system.
 26. The fuel processor accordingto claim 15, further comprising: a catalyst combustor positioned suchthat an output of said catalyst combustor is input downstream of saidsecond combustion heater system.
 27. A fuel processor comprising: areformer converting a hydrogen-containing fuel selected from the groupconsisting of alcohols and hydrocarbons to H₂-containing reformate; ashift reactor in fluid communication with said reformer, said shiftreactor being operable to reduce carbon monoxide levels of saidreformate; a preferential oxidation reactor in fluid communication withsaid shift reactor, said preferential oxidation reactor being operableto further reduce carbon monoxide levels of said reformats exiting saidshift reactor; a first combustion heater system coupled to saidreformer, said first combustion heater system operated in a lean stateto produce thermal energy as a product of internal combustion in theform of a first heated exhaust stream that is passed through saidreformer; and a second combustion heater system coupled to said shiftreactor, said second combustion heater system operated in a near idealstoichiometric condition to produce thermal energy as a product ofcombustion in the form of a second heated exhaust stream that is passedthrough said shift reactor.
 28. The fuel processor according to claim27, wherein said second combustion heater further comprises an air inletproviding fresh air thereto.
 29. The fuel processor according to claim27, wherein said second combustion heater system is positioned in serieswith said first combustion heater system.
 30. The fuel processoraccording to claim 29, wherein said second combustion heater furthercomprises a first inlet providing fresh air thereto and a second inletin fluid communication with said first heated exhaust stream.
 31. Thefuel processor according to claim 30, wherein said first heated exhauststream is used to dilute said fresh air to control the temperature ofsaid second heated exhaust stream.
 32. The fuel processor according toclaim 29, further comprising: a first control valve system selectivelyrouting said first heated exhaust stream to said second combustionheater system during a startup cycle.
 33. The fuel processor accordingto claim 27, further comprising: a spray vaporization zone coupleddownstream from said second combustion heater system, said sprayvaporization zone being operable to maintain a predeterminedtemperatures of said second heated exhaust stream.
 34. The fuelprocessor according to claim 27, further comprising; a second controlvalve system selectively routing an O₂-containing cathode effluent froma fuel cell stack to a catalyst combustor arid said second combustionheater system.
 35. The fuel processor according to claim 27, furthercomprising: a catalyst combustor positioned in series upstream from saidsecond combustion heater system.
 36. The fuel processor according toclaim 27, further comprising: a catalyst combustor positioned in seriesdownstream from said second combustion heater system.
 37. The fuelprocessor according to claim 27, further comprising: a catalystcombustor positioned such that an output of said catalyst combustor isinput downstream of said second combustion heater system.